The present invention relates to an engine to be mounted in a vehicle equipped with a heater that uses engine coolant as a heating source, and, more particularly, to a combustion control apparatus and combustion control method for an engine that carries out stratified-charge combustion (stratified combustion).
An engine has been proposed that carries out stratified combustion, in which fuel is non-uniformly injected into the combustion chamber of the engine near the ignition plug, and homogeneous-charge combustion (homogeneous combustion), in which fuel is uniformly distributed in the combustion chamber (see Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-189405, for example).
Generally, during the stratified combustion, the temperature of the combustion gas that contacts the inner wall of the combustion chamber is relatively low, thus reducing the amount of heat transferred to the engine coolant through the inner wall. This makes the cooling loss lower than that of homogeneous combustion. Since the air-fuel mixture is leaner during stratified combustion, the throttle valve is opened more widely, accordingly. This makes the pumping loss lower than that of homogeneous combustion.
Because of the reductions in cooling loss and pumping loss, stratified combustion reduces fuel consumption as much as possible while providing about the same engine power as that provided by the homogeneous combustion.
While an engine that carries out stratified combustion consumes less fuel, it raises the following significant problem when used in a vehicle equipped with a hot water heater for heating the interior of the vehicle.
Since the rate of heat transferred to the coolant is lower during stratified combustion, the temperature of the coolant also becomes lower than that of homogeneous combustion. Because the hot water heater uses this coolant as the heat source, the heat output may be inadequate.
In particular, if stratified combustion is performed in a low-engine-load mode, such as when idling, which relatively reduces the amount of fuel injection as described in the aforementioned Japanese publication, the amount of combustion heat generated in the combustion chamber is smaller. This further reduces the heat output of the heater.